1. Field of the Invention
The present invention relates to circuit breakers and, more specifically, to a slot motor for a circuit breaker wherein the slot motor has two legs separated by a gap and the gap has a narrow portion and a stepped out portion.
2. Background Information
Circuit breakers, including molded case circuit breakers, have at least one pair of separable contacts. A first contact is fixed within the molded case housing and the other contact, the xe2x80x9cmovable contact,xe2x80x9d is coupled to an operating mechanism. Both contacts are disposed on xe2x80x9carmsxe2x80x9d that are in electrical communication with either the line or load coupled to the circuit breaker. The operating mechanism is structured to move the movable contact between a first, open position wherein the movable contact is spaced from the fixed contact, and a second, closed position wherein the fixed and movable contacts are in contact and electrical communication. The operating mechanism may be operated manually or by the circuit breaker""s trip mechanism. To enhance the speed of separation of the contacts, the contacts may be disposed within a slot motor.
A slot motor is a ring or loop-shaped device made of magnetically permeable material which surrounds the contacts and contact arms of a circuit breaker. When the circuit is live, an electrical arc may be drawn between the electrical contacts during separation. The electrical current interacts electromagnetically with the slot motor to induce a magnetic field in the magnetic material of the slot motor which in turns interacts with the separating contact arms to accelerate the contact opening process. An example is found in U.S. Pat. No. 4,970,482 issued Nov. 13, 1990 to Jacobs et al, entitled xe2x80x9cCurrent Limiting Circuit Breaker Arc Chute Configurationxe2x80x9d.
Slot motors generally have two assemblies, an upper assembly and a lower assembly. Both upper and lower assemblies include a housing and a plurality of plates composed of the magnetically permeable material. The lower assembly is disposed below the fixed contact. As shown in FIG. 1, the upper assembly is an inverted U-shaped assembly having a housing assembly 1 and a plurality of plates 2, forming a bight portion 3, two legs 4, 5. The upper slot motor is structured to be disposed over the movable contact wherein the tips of the upper assembly leg contact the lower assembly. The legs of the U-shaped upper assembly have an extended length to accommodate the path of travel of the movable contact arm. That is, the movable contact is disposed between the legs of the upper assembly and as the movable contact moves between the first, open position and the second, closed position, the movable contact moves from a position adjacent to the upper assembly bight to a position adjacent the tips of the legs. Accordingly, the legs must have a sufficient length to accommodate the path of travel of the movable contact arm. It should further be noted that the movable contact arm may, due to manufacturing tolerances, be free to shift a short distance laterally while moving.
The prior art slot motors have a generally uniform gap between the legs of the U-shaped upper assembly. In other words, the inner surfaces 6, 7 of the legs 4, 5 adjacent to the movable contact, are generally parallel. Moreover, the inner surface 6, 7 of each leg is typically within 0.6 inch of the movable contact. Such prior art slot motors are subject to failure due to carbon build up which creates a voltage path when the contacts are in the first, open position. That is, when the contacts are opened and an arc created, carbon is deposited on the upper assembly inner surface. Over time, the carbon builds up and eventually allows voltage to travel from the movable contact through the carbon to the fixed contact. The chance of a voltage path being created is enhanced when the movable contact arm shifts laterally toward either leg inner surface 6, 7. This situation is unacceptable as current may flow through the circuit breaker even after the circuit breaker has tripped or has been manually opened.
There is, therefore, a need for a slot motor structured to prevent a voltage path from forming on the surface of the slot motor.
There is a further need for the improved slot motor to be compatible with existing circuit breaker housings.
These needs, and others, are met by the present invention which provides a slot motor having an inner surface with a stepped out portion. The outer surface of the slot motor maintains the same, U-shaped profile of the prior art slot motor. As such, the slot motor of the present invention may be used in prior art circuit breakers. The inner surface on each leg or the slot motor has an inset portion and a stepped out portion, or, stated alternately, the gap between the legs of the upper slot motor has a narrow portion and a wide portion. The inset, or narrow gap, portion is disposed adjacent to the distal end of each leg and the stepped out, or wide gap, portion is disposed adjacent to the bight of the U-shaped slot motor. The gap between the inset portions, the narrow gap, is generally about 0.256 inch and the gap at the stepped out portion, the wide gap, is generally about 0.356 inch.
In operation, when the circuit breaker contacts are closed, the contacts are disposed between the inset portions of the slot motor which corresponds to the narrow portion of the gap. Thus, when the contacts begin to separate, the slot motor is generally within 0.033 inch of the contacts. The proximity of the contacts to the slot motor upper slot motor assembly inner surface at the inset portion enhances the strength of the magnetic field created by the arc during the initial separation arid therefore enhances the speed of separation. As the movable contact moves into the stepped out portion of the slot motor, the gap between the legs widens. At the end of the separation, the movable contact is disposed in the stepped out portion. Thus, the movable contact is disposed about 0.053 inch from the inner surface while in the first, open position. This gap is sufficient to prevent a voltage path from forming along the inner surface of the slot motor, even when the inner surface is coated with carbon.